A liquid crystal display panel is constituted by two opposing substrates between which liquid crystal is enclosed. FIG. 7 is a schematic cross-sectional view of an exemplary color liquid crystal display panel, which is one of liquid crystal display panels. Liquid crystal 30 is enclosed as interposed between a TFT (Thin Film Transistor) substrate 1 and a CF (Color Filter) substrate 5. On a main surface of TFT substrate 1, a scan line drive circuit 2 is formed and connected to an external drive IC (not shown). On a main surface of scan line drive circuit 2, a pixel electrode layer 3 is formed. In pixel electrode layer 3, in addition to a pixel electrode, a TFT is also formed. On a main surface of pixel electrode layer 3, an alignment film 4 is formed.
On a main surface of a CF substrate 5 opposing to TFT substrate 1, a color filter 6 is formed. Color filter 6 has the hue of three; red, green and blue. On a main surface of color filter 6, a common electrode 7 is formed. On a main surface of common electrode 7, an alignment film 8 is formed. Liquid crystal 30 is charged to be interposed between alignment film 4 and alignment film 8, and a sealant 31 is arranged on the side of liquid crystal 30. That is, liquid crystal 30 is enclosed so as to be surrounded by alignment films 4 and 8 and sealant 31. A certain space is kept between alignment films 4 and 8 by a spacer 9 interposed between them. Thus, liquid crystal 30 directly contacts to alignment films 4 and 8 and sealant 31. The two substrates are fixedly adhered to each other by sealant 31. In the present specification and the claims, a single substrate such as a TFT substrate, and a single substrate wherein a pixel electrode layer, an alignment film and the like are formed are both simply referred to as “a substrate”.
Among manufacturing methods of a liquid crystal display panel, there is a manufacturing method referred to as liquid crystal dropping scheme. FIG. 8 shows steps of the liquid crystal dropping scheme. First, a color filter and a pixel electrode layer are respectively formed on two substrates, and thereafter an alignment film is formed on each of the substrates. Next, a sealant is arranged on a main surface of one of or each of the alignment films respectively formed on the TFT substrate and the CF substrate. Next, liquid crystal is dropped on the main surface of one of the substrates' alignment films by a required amount. FIG. 9 is a schematic cross-sectional view wherein the two substrates are bonded to each other in accordance with the liquid crystal dropping scheme. In FIG. 9, components such as the spacer and the color filter other than alignment films 4 and 8 are not shown. In bonding the substrates as shown in FIG. 9, sealant 31 is arranged on the main surface of alignment film 8 formed on CF substrate 5, and liquid crystal 30 by a required amount is dropped on the main surface of alignment film 4 formed on the opposite TFT substrate 1. In bonding the substrates, as indicated by arrow 50, respective main surfaces of the two substrates are kept in parallel to each other and drawn toward each other to be fixedly adhered. The bonding of the two substrates is performed in a pressure-reduced atmosphere. The arrangement of the sealant and the dropping of the liquid crystal after the formation of the alignment films on substrates are performed in a released atmosphere, and the bonding of the substrates alone is performed in a pressure-reduced atmosphere.
The alignment films are made of a porous material, and therefore their surfaces easily adsorb moisture. If moisture is mixed into the liquid crystal, a problem arises that a voltage holding ratio decreases and that an image is unevenly shown or shown with spots. In order to prevent moisture from mixing into liquid crystal as a result of the moisture having been adsorbed by the surfaces of alignment films and the like, the manufacturing process includes a step of arranging the substrates in a pressure-reduced atmosphere for a certain period, which is referred to as a deaerating step. In order to carry out the deaerating step, a vacuum chamber, a vacuum pump and the like are necessary for reducing the pressure around the entire substrates. In order to successively carry out the steps from the arrangement of the sealant to the bonding of the substrates in a pressure-reduced atmosphere, a facility of enormous size is required. Additionally, if the deaerating step is carried out prior to the arrangement of the sealant, the substrates after the deaerating step are stored in a released atmosphere from the arrangement of the sealant until the bonding of the substrates. During this period, moisture in the air may again be adsorbed by the main surface of the alignment film, for example. Accordingly, the deaerating step is carried out immediately before bonding the two substrates to each other. The other steps such as arranging the sealant, dropping the liquid crystal, transporting the product and the like are generally carried out in a released atmosphere.
Japanese Patent Laying-Open No. 2003-107481 discloses a manufacturing method of a liquid crystal display panel wherein water adsorption of an alignment film immediately before arrangement of a sealant is set to be less than 4 weight %, and each step after formation of the arrangement films and at least until completion of bonding is carried out in an environment of humidity of less than 40%. In the manufacturing method, for example, after the alignment films are formed, storage or assembly is conducted in an environment wherein humidity is less than 40%, and storage or assembly is conducted while dry air is purged onto the substrates. The step of arranging sealant on the surface of a substrate is performed in an atmosphere of dry air, for example, and the steps of dropping liquid crystal and bonding the substrates are performed in an atmosphere of vacuum. According to this method, moisture is prevented from being adsorbed by the alignment films in storing or assembling the substrates, and therefore a liquid crystal panel not involved with residual images can be manufactured.
Japanese Patent Laying-Open No. 2001-305545 discloses a manufacturing method for preventing moisture or adsorptive gases from being adsorbed by alignment film surfaces by inactivating the alignment film surfaces immediately after a rubbing process. According to this method, a heating process is performed on the alignment film surfaces immediately after the rubbing process at the temperature of at least 80° C. and less than 100° C. for a prescribed period to thereby inactivate the polarity of the alignment film surfaces. Thus, moisture or adsorptive gases are prevented from being adsorbed by the alignment film surfaces, whereby various unevenness in display can be prevented.
Patent Document 1: Japanese Patent Laying-Open No. 2003-107481 (pages 3-5, FIGS. 2-5)
Patent Document 2: Japanese Patent Laying-Open No. 2001-305545 (pages 3 and 4)